st: Extracting data from mixed string

["Paul O'Brien" <paul.obrien@afe2.org.uk>]

Database of 41,000 clinic visits with 26 variables for each visit. One 
variable  is a string (stk) of contraceptives prescribed, containing 1 
to 4 types, separated by commas.

stk
NONE,OVYS
NONE,NONE,NONE,MATES6,NATURAL
NONE,MICROG
NONE,NONE
NONE,FEMODE,NATURAL
NONE,NONE
LOGED
NONE,NONE,MICROG,MATES6,NATURAL
NONE
NONE,LEVONEL,MICROG
MICROG
NONE,NONE,LOG
NONE,NONE,NONE
NONE,MATES6,NATURAL
NONE,NONE
MICROG,ULTRA
NATURAL
NONE,MICROG,NATURAL

Some are oral contraceptives:OVYS   MICROG  FEMODE, LOGED

The position of the oral contraceptives in the string varies.

I want all the oral contraceptives in one column:

OVYS
blank
MATES6
MICROG
blank
FEMO
blank
LOGED

How?

Help gratefull received.

Paul

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